AFAIK Carsten intends to switch back to GTK+2 for Jessie but it would be great if you could go hunting for patches since we might be forced to switch to GTK+3 at some point in the future.
It could be a hard work to figure out all issues related to GTK3. It could be easier to determine if GTK2 will be dropped in the next 3 years (remaining lifetime of Debian Jessie if I remember correctly).
Best regards. -- Alex ARNAUD Visual-Impairment Project Manager Hypra - "Humanizing technology"